In many jurisdictions, one or more ranges within the radio frequency (RF) spectrum are not regulated by a licensing regime. For example, with the transition to digital television by the United States in June 2009, a substantial amount of RF spectrum previously dedicated to licensed analog television broadcasts has been made available to unlicensed users, and these newly available RF spectrum channels are referred to collectively as Television White Spaces (TVWS). Although some regulation remains for the unlicensed spectrum, various standards bodies have targeted the unlicensed spectrum in efforts to manage communications within these frequency bands.
Accordingly, wireless protocols in the unlicensed spectrum have been developed for different requirements in terms of range and power. However, the different parameters of such wireless protocols make it difficult for communications of different protocols to coexist in the same unlicensed channel. For example, intentionally low-power Zigbee® nodes are frequently starved by higher-power WiFi® nodes. Furthermore, as various incompatible standards become more accepted and their networks become more widely distributed, the challenges of coexistence among wireless networks will become more severe.
Disparate power levels particularly contribute to these challenges. For example, in many wireless communication protocols, nodes “listen” for other communications within their operating RF frequency spectrum. If the first node detects that another node is transmitting at that time (e.g., carrier sensing), the first node “backs off” for a short period of time (e.g., collision avoidance) and tries again. However, disparate transmission power levels between the nodes translate to different transmit ranges (e.g., the transmit range is the distance from which a packet can be transmitted from one node and received and decoded by another node). As such, a higher power node may sit outside the standard transmit range of a lower power node and therefore not detect the transmissions of the lower power node, although the lower power node can detect the transmissions of the higher power node. In this example, since the higher power node does not “hear” the lower power node, the higher power node may unwittingly transmit at a time that interferes with transmission of the lower power node. The throughputs of both nodes are diminished during periods of such interference.
Further, the lower power node can detect the transmissions of the higher power node and will therefore back off if the lower power node detects a transmission of the higher power node. However, the same is not true for the higher power node—because the higher power node does not detect transmissions of the lower power node, the higher power node does not back off. As such, the unrestrained transmission of the higher power node effectively starves the lower power node of bandwidth.